The present invention relates generally to the bonding of two crystals of material together and more particularly is concerned with a method of bonding together two crystal components to form an acoustical-optical device. Acoustical-optical devices are generally comprised of a piezoelectric crystal bonded to an optical diffraction crystal. The bond between these two types of crystals must have good mechanical strength, controlled thickness and also must be formed of a material which will effectively transmit acoustical energy.
It is known in the prior art to bond crystals together by the use of low melting point alloys and pressure fusing under relatively low temperatures. For example, U.S. Pat. No. 3,252,722 to Allen, discloses the deposition of a layer of indium on one crystal and a layer of gold on the opposite crystal and thereafter, pressure bonding or fusing the gold to the indium. Another technique for bonding is disclosed in U.S. Pat. No. 3,921,885 to Knox wherein, each of the crystals is first coated with a chromium film and thereafter each is coated with aluminum film, thereover; following which each is coated with a gold film. Thereafter, pressure is applied to cause a bonding of the two gold layers.
Each of these two patents has several serious limitations. The Allen patent has the problem of oxidation of the layer of indium and thus if there is any exposure to the air, the indium surface must be burnished as is suggested therein with a clean nylon parachute cloth. This is a mechanical operation and constitues the physical removal of an oxide film which film is unwanted and deleterious to bonding. Also, there is associated with this technique, the necessity of regulating the amount of burnishing to assure that proper surfaces and thickness are provided.
The Knox patent requires very high pressures and the quality of the bonds is not always at the desired level. Further there is no low melting point alloy used to provide the desired bond which is disclosed in the Allen patent teaching.
Other teachings relate to the manufacture of crystals using soldering type modes, e.g., U.S. Pat. No. 3,111,741 to Allen, and high temperature bonding such as shown in U.S. Pat. No. 2,618,579 to Brajer. These techniques also have many limitations.